doc: iio: ad7606: describe offload support

Add a section to the ad7606 documentation describing how to use the
driver with SPI offloading.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250403-wip-bl-spi-offload-ad7606-v1-2-1b00cb638b12@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>